(1) Field of the Invention
This invention relates to a circuit for increasing the inductance of a transformer. More particularly the invention relates to a stable inductance multiplier for use with a line circuit battery feed inductor including an arrangement for compensating for resistance changes of said inductor.
(2) Background Art
Battery feed for a telephone traditionally has been supplied through split transformer windings coupled by a DC blocking midpoint capacitor. Because the split windings must carry significantly large DC currents the core of the transformer must be fairly large to keep it from saturating. This technique of supplying battery works very well particularly because of the large degree of transient protection afforded between transformer windings. The major drawback lies in the large physical size and expense of the transformer.
One method used to reduce the size of the transformer is to multiply its inductance by placing an electronically simulated negative inductor in parallel with a tertiary winding. This technique allows the initial pre-multiplied inductance of the transformer to be considerably smaller. Thus, fewer turns passing a direct current are required and a smaller transformer can be used. The electronic negative inductor is easily protected from high voltage transients because its only coupling to the line side of the transformer is inductive.
The use of an op-amp circuit to simulate an electronic negative inductor which is placed in parallel with a transformer tertiary winding to increase the inductance seen looking into all the other windings has the problem of stability. The op-amp circuit has an input impedance equivalent to a negative inductor in series with a negative resistor. In order to guarantee circuit stability no matter what impedance is connected to the line side of the transformer the loop containing the negative resistor and negative inductor has to be compensated for with a positive resistance. This positive resistor has to be exactly equal to the simulated negative resistor. If this is accomplished the circuit will not oscillate and it also will not load down the input impedance seen looking into the other transformer windings. However, this positive resistance includes the nonzero resistance of the tertiary winding. This coil resistance varies greatly with temperature because of the extremely large temperature co-efficient of copper. Also if the positive resistance becomes greater than the negative resistance at DC the op-amp circuit will latch up at either the positive or negative rail. Thus trying to match the positive and negative resistance with a factory tuned potentiometer is impractical since if the temperature of the transformer increased, its winding resistance would increase and the negative inductor would latch up.